Bill Gates, Jeff Bezos, Mark Zuckerberg, and a host of their uber-rich compatriots are investing in two energy startups that could solve one of the biggest problems with renewable energy.

They are investing through a $1 billion fund, Breakthrough Energy Ventures, known as BEV, which announced last year that energy storage would be one of its main focuses.

Launched in 2016, the fund is also supported by LinkedIn's founder, Reid Hoffman; Alibaba's CEO, Jack Ma; and David Rubenstein, the executive chairman of the private-equity giant Carlyle Group.

Part of BEV's mission is to provide "patient capital." That means BEV is willing to forgo returns on investment for up to 20 years to give the scientists and engineers at startups a reasonable lead time to develop world-changing technologies.

The two startups have developed different techniques to store energy — one of the biggest bottlenecks in the widespread adoption of renewable-energy sources.

Quidnet Energy's technology uses water to store energy in a novel way. Using excess electricity, Quidnet pumps water into repurposed or unused oil-and-gas wells within shale rock formations. When the liquid is pumped in, the water fills cracks in the rock, creating pressure.

To generate electricity, Quidnet releases the water, and the pent-up pressure is used to run turbines and generate electricity.

Form Energy — which counts an MIT professor and a former Tesla engineer among its leadership — is working on developing a super-efficient battery that could store large amounts of energy for long periods of time.

Batteries' costs are generally measured by how much it costs to store a kilowatt-hour of energy. Form Energy is developing battery technology that could cost as little as $10 per kWh — the cheapest batteries today are about $200 per kWh, Quartz reports.

That ultimately has the potential to bring down the cost of electric vehicles and to extend their range. It could also make it more cost-effective to power homes with solar energy.